1. Technical Field
The present invention relates to a turbo compound engine in the form of a highly supercharged engine that is equipped with either a turbocharger or a supercharger (referred to as "turbocharger" hereinafter) and a power turbine. In particular, it concerns a turbo compound engine that is capable of developing an engine brake force that is at least equal to that of a non-supercharged engine of equal power output.
2. Background Art
Generally, supercharged engines feature in comparison with non-supercharged engines of larger displacements: (1) better fuel economy, (2) output performance that is at least equal to non-supercharged engines, and (3) lighter weight and compactness of the engine.
These advantages of supercharged engines are further augmented in the highly supercharged engines and in turbo compound engines, and this invention is especially concerned with the latter.
In the turbo compound engine, as shown in FIG. 11 of the accompanying drawings, the energy of exhaust gas from the engine b is recovered by the turbocharger c as its supercharging work, and the remaining energy of the exhaust is recovered by the power turbine d as its power work. Although this construction brings about a general improvement in the power output performance, fuel economy, and gain of the engine b, the expansion ratios of the turbocharger c and the power turbine d must be increased in order to further improve the turbo compound engine's overall performance. Specifically, raising of supercharging pressure mandates an increase in the effectiveness of a turbo compound engine.
A typical prior art engine of the type mentioned above is disclosed in Japanese Utility Model Laid Open No. 157,941/85. In this engine, there is connected, as shown in FIG. 12 of the accompanying drawings, to the exhaust passage e that connects the turbocharger c.sub.1 and the power turbine d.sub.1, a bypass f that bypasses the power turbine d.sub.1, and there is disposed in the junction g of exhaust passage e and bypass f a switching valve h, which functions to throttle off the exhaust passage e in proportion to the depression of the accelerator pedal (not shown) of the engine.
It is an object of the present invention to reduce diminishment of power output performance when the energy of exhaust is low caused by the back pressure acting on the turbine of the turbocharger c.sub.1 by letting the exhaust bypass through the bypass f.
The magnitude of the flow rate of exhaust, and hence the energy thereof, is sensed in terms of the magnitude of depression of the accelerator pedal. A problem exists in using this type of internal combustion engine in a vehicle, in that obtaining an engine brake (exhaust brake) force commensurate with increased power output is difficult. This may be seen in the relationships existing among engine revolution speed, engine output power Pme (solid lines), and engine brake force Pmf (broken lines), shown in FIG. 13 for a highly turbocharged engine in comparison with a non-supercharged engine. Specifically, with regard to the relative braking force (i.e., engine revolution speed/engine output power) at the 100% rated revolution speed N.sub.100, B.sub.N /S.sub.N &gt;B.sub.T /S.sub.T, where B stands for the engine brake force, S the engine output power, the subscript N stands for a non-supercharged engine, and T stands for a turbocharged engine. This means that the relative engine brake force suffers a decrease when the supercharging is intensified.
To reiterate, obtaining a sufficiently large engine brake force is important not only for the maneuverability and safety of the vehicle, but also for taking better advantage of the advantages of the turbo compound engine.